Recently, a technique for realizing a high-density recording has been developed as a result of fusion of optical technology and magnetic recording/reproducing technology. For example, Patent Document 1 discloses a magnetic recording medium made of a ferromagnetic substance whose compensation temperature is substantially a room temperature, and a heat-assisted magnetic recording/reproducing method in which recording/reproduction of information is carried out, by using a laser beam, with respect to the magnetic recording medium made of the ferromagnetic substance.
In a magnetic recording/reproducing device that utilizes a heat-assisted recording/reproducing method as disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 176034/1992 (Tokukaihei 4-176034) (published on Jun. 23, 1992)), a temperature of the magnetic recording medium is increased by a laser beam at the time of recording so as to decrease a coercive force of a recording area in the magnetic recording medium. While the coercive force of the recording area in the magnetic recording medium is in a decreased state as described above, information is magnetically recorded on the magnetic recording medium by applying, with the use of a recording head, an external electric field to the magnetic recording medium.
Meanwhile, at the time of reproduction, temperature dependency of remanent magnetization is utilized. A temperature of a reproduction area in the magnetic recording medium is increased by the laser beam to a temperature at which the remanent magnetization becomes sufficiently large. Then, information recorded on the magnetic recording medium is reproduced by detecting, with the use of a reproducing head, a magnetic flux from the remanent magnetization that has become sufficiently large by the increased temperature. Here, in a reproduction area whose temperature is not increased by the laser beam, the remanent magnetization is close to 0. Therefore, crosstalk due to a signal that leaks from an adjacent track can be suppressed to a sufficiently small level. This makes it possible to realize the reproduction of the information that is recorded in a high density.
In the magnetic recording/reproducing device that utilizes the heat-assisted magnetic recording/reproducing method, the temperature of the recording area or the reproduction area of the magnetic recording medium affects stability of recording/reproduction and a signal quality. Therefore, it is important to control heat generation means for heating the magnetic recording medium.
For example, Patent Document 2 (Japanese Unexamined Patent Publication No. 298301/2002 (Tokukai 2002-298301) (published on Oct. 11, 2002)) discloses a control method of heat generation means for heating a magnetic recording medium. The control method includes steps of: writing in predetermined specific data; reproducing the written-in data; comparing a read result of the reproduced data with the specific data to be written in; and recording, in a memory, a temperature inside a device and control information of heat generation means which are of a case where recording is successful, as a reference at the time of controlling the heat generation means.
However, the following problem occurs in a case, as in the conventional recording/reproducing method as disclosed in Patent Document 1 and the conventional heat-assisted magnetic recording/reproducing method as disclosed in Patent Document 2, that employs a compound semiconductor light source such as a semiconductor laser or a light emitting diode. This problem is explained by taking a semiconductor laser as an example of the compound semiconductor light source.
The semiconductor laser has a driving current that causes the semiconductor laser to perform laser oscillation, that is, a threshold current. However, the threshold current is known to vary depending on a temperature. Accordingly, even if, as in the heat-assisted magnetic recording/reproducing method as disclosed in Patent Document 2, the temperature inside the heat-assisted magnetic recording/reproducing device and the control information of the heat generation means are recorded, for every trial recording, in a memory inside the heat-assisted magnetic recording/reproducing device, the temperature momentarily varies due to driving of the device. Therefore, unless a vast number of trial recordings are made, the device cannot deal with the variation in the threshold current. The following explains further in detail by taking one example.
First, the following assumptions are made. That is, an operation temperature range is in a range of, for example, −20° C. to +85° C. Moreover, a wavelength of a semiconductor laser used as a light source of the device is 658 nm. Further, a threshold current in a case of a temperature of 25° C. is 25 mA. A temperature dependency Δt of the threshold current is 0.3 mA/° C. A slope efficiency (a relationship between a driving current and a light output after laser oscillation) is 0.55 mW/mA.
On such assumptions, in a case where the temperature inside the device increases from 25° C. to 30° C. by 5° C., the threshold current of the semiconductor laser increases by 1.5 mA. Accordingly, in a case where the semiconductor laser is driven by constant current driving, the light output decreases by 0.83 mW. Meanwhile, the light output necessary for recording a signal on the magnetic recording medium decreases only by 0.1 mW. This is based on the following calculation.
For example, the followings are assumed: the temperature inside the device is 25° C.; and a temperature of a magnetic recording medium is 200° C. at the time of recording a signal. If the temperature of the magnetic recording medium at 25° C. that is the same as the temperature inside the device can be increased to 200° C. by heating with the use of the light output of 3 mW from the semiconductor laser, an amount of increase in the temperature of the magnetic recording medium decreases from 175° C. to 170° C. (−3% with respect to 175° C.) in a case where the temperature inside the device is 30° C. Therefore, in simple calculation, the light output of the semiconductor laser necessary for the increase in the temperature of the magnetic recording medium becomes 2.9 mW that is −3% with respect to 3 mW. From thus obtained result, the light output necessary for recording a signal on the magnetic recording medium decreases only by 0.1 mW.
In other words, in a case where the semiconductor laser is driven at a constant driving current regardless of temperature variation inside the device, the light output of the semiconductor laser decreases at a rate that is larger than a rate at which the light output necessary for recording a signal on the magnetic recording medium decreases. Accordingly, shortage occurs in the light output from the laser, for recording a signal on the magnetic recording medium. That is, due to an insufficient temperature increase of the magnetic recording medium, desired recording or a reproduction quality cannot be maintained.
As explained above, a light output of a compound semiconductor light source such as a semiconductor laser and a light emitting diode has temperature dependency. Therefore, if control information of a heat generating device is sought from trial records every time a user tries to record a signal on the magnetic recording medium, it takes a lot of time for trial recordings. Moreover, the light output at the time of starting the trial recording may largely differ from an optimum condition for carrying out recording on the magnetic recording medium. In a case where the light output at the time of starting the trial recording is excessively larger than that in the optimum condition for carrying out recording on the magnetic recording medium, user data may be wrongly erased from the magnetic recording medium.
A possible method to deal with the problem caused by the temperature dependency of the light output from the compound semiconductor light source is a method to deal with variation in the light output according to temperature variation by receiving, with the use of a light receiving element, the light output from the compound semiconductor light source. However, recently, a size of a recording/reproducing head has been further reduced. Therefore, in a case of a recording/reproducing head that is integrated with the compound semiconductor light source, it may be difficult to mount a light receiving element on the recording/reproducing head.